web: fix overflow glitch on ak-page-header (#7883)

By adding 'grow' but not 'shrink' to the header section, the page was allowed to allocate
as much width as was available when the window opened, but not allowed to resize the width
if it was pushed closed by zoom, page resize, or summon sidebar.

This commit adds 'shrink' to the capabilities of the header.

web/src/elements/PageHeader.ts

@@ -79,6 +79,7 @@ export class PageHeader extends AKElement {
                 }
                 .pf-c-page__main-section {
                     flex-grow: 1;
+                    flex-shrink: 1;
                     display: flex;
                     flex-direction: column;
                     justify-content: center;